1. Field of the Invention
This invention relates to the detection of leaks in underground pressurized piping, used for fluid transmission of hydrocarbon fuels, and other incompressible hazardous fluids. Federal and State environmental protection regulatory laws, mandate such leak detection devices.
2. Description of Prior Art
A myriad of devices have been available, attempting to perform line leak detection, on underground pressurized piping. These devices break down into categories of mechanical and electro-mechanical designs. Leak rate determination breaks down into categories of pressure decay and flow.
With the advent of the micro-computer and associated circuitry, the electro-mechanical designs, have taken on a sophistication heretofore not available. The intelligent circuitry permits a level of decision making that has made available leak detection rates that were previously impossible.
Prior to intelligent circuits, leak detectors sensed the time to pressure up or the time to pressure down, with pump turn on or off, and alarmed when the preset value was exceeded. These devices had no capability to discriminate between effects caused by leaks and thermal expansion or contraction. Additionally deleterious effects caused by trapped air or piping springiness were ignored.
New Federal and State requirements for leak detection specify leak rate detection at leak rates, beyond the capability of the old traditional designs. The new designs utilizing intelligent circuits, employ two fundamental approaches for leak detection, pressure decay or flow.
The pressure decay devices measure the pressure drop to a predetermined level over time, and compensate for temperature change, by taking temperature readings or changes of time to a predetermined pressure level, by taking multiple tests. Limits are set for temperature or time changes, when exceeded a no test is signalled. The effects from air or piping springiness, is compensated for, by setting standards to typical values for both.
The flow devices measure any flow detected when pumping pressure is supplied for a predetermined time interval against a closed nozzle. Various strategies are employed to determine a no test condition caused by out of limit temperature differentials. The effects from air or piping springiness is compensated for by the constant pressure applied during the test cycle.
Strategies employed by existing pressure decay or flow devices, concerning the effects from a temperature differential between fluid and soil, that cause fluid expansion or contracting, do not eliminate the effect, but only declare a no test condition, when the effect is beyond a prescribed limit. The result of this circumstance may be many no test status results, usually during extreme weather conditions.
The springiness of a piping system or air in the system, can be represented by the system bulk modulus value. The piping systems relative bulk modulus, can be determined by the fluid volume release, from any test pressure. For any fuel piping system in continuous use, the bulk modulus is usually constant.
For a pressure decay detector the effects of the piping bulk modulus, can be easily compensated for, because the bulk modulus is always a constant value, for piping in continuous use.
For a flow detector the effects of the piping bulk modulus, is automatically compensated for, by the constant pumping pressure applied during the flow test, for a leak.
A temperature condition refers to the difference of temperature, between the fluid in the pipe and the surrounding soil. When the fluid is hotter than the pipe, pressure is lost over time. When the fluid is colder than the pipe, pressure is gained over time. The primary fluid is gasoline, the coefficient of thermal expansion for gasoline fuels is 6 to 7 times that of water, small temperature changes can result in large pressure changes. These changes can result in false leak detection reporting, in both directions. Both pressure decay and flow measurement devices, are sensitive to temperature differentials of the piping fluid and the soil. Motor fuels are stored primarily in underground tanks, often fuel in the tank is a different temperature, than the piping, that delivers fuel to the dispenser.
Temperature differentials of less than 1 degree centigrade, can cause pressure fluctuations as great as 10 psi, in pressurized piping.
Pressure decay devices that utilize sequential pressure-up cycles, will introduce heat into the piping fluid under test.
Likewise flow devices that utilize constant pressure, from pump operation, are additionally likely to introduce heat into the piping fluid.
The source of heat is due to fluid hysteresis, caused by the pulsation of the fluid in the piping. The design of submersible pumps turbine wheel, creates the fluid pulsation, thereby introducing heat into the fluid, under pressure.
Pressure decay devices, depend on the turbine pumps, combination check/pressure-relief valve, to capture the fluid under test, with a pressure drive. Usually the pumps operating pressure is 20 to 30 psi and the pumps off pressure is 8 to 12 psi. Generally for a 3 gph test, any post pressure held above 4 psi for 10 seconds is declared a pass. For this test the effects of temperature differentials, are removed, due to short time of test. The test is said to fail within the adiabatic regime. For a precision test 0.1 to 0.2 gph, the test must be for a significantly longer duration. For a normal bulk modulus and for a 0.2 gph test, a 5-10 minute duration test is usually required. With a relatively low starting test pressure, that will decrease as fluid is released, sensitivity to the fluid/piping temperature differentials, can create false leak reporting. Existing strategies are to record temperature gains or losses, during the test, or to perform sequential tests looking for a time differential to a predetermined pressure setting. Sequential tests, requiring pump operation, risks fluid heating from turbine pump pulsation. Temperature recordation during the test, is subject to sensor location along the length of the piping. A single sensor may miss a temperature differential, somewhere along the length of the piping system. Pressure decay leak detection devices test the residual pressure, trapped in the piping system after pump shut-off. Submersible turbine pumps are equipped with valves, that acts as a check against pressure loss, but also as a pressure relief valve. It is necessary to trap pressure in the line from pump to dispenser, to prevent draining of the line, draining of the line would allow air into the line. Air trapped in the pump to dispenser line, would cause an incorrect metering of product through the dispenser. The pressure relief valve is required, to prevent over pressurizing the line, from product expansion on a hot day. Usual pumping pressures of a new pump, against a closed nozzle is 30 psi, while pump off pressures are 10 psi.
Some submersible pump manufacturers supply pumps, with 30 psi pumping pressures and 45 psi pressure relief valves. Since the higher relief pressure is seldom reached, a lower cost valve can be used. The majority of pumps operate at 30 psi and 10 psi off, most pressure decay devices are compatible with that range. Pressure decay leak detectors rely upon the pumps valving to successfully relieve pressure to the nominal 10 psi. For the 3 gph test, nominally, if the trapped pressure remains above 4 psi, for 10 seconds, the test is declared a pass. For the 0.1 or 0.2 gph test, nominally, if the trapped pressure remains above 7 psi for 5 to 10 minutes, the test is declared a pass, providing a temperature condition is not sensed. Should the pumps valving fail to trap pressure, a false leak will be declared. Should the pumps valving fail to trap pressure, always at the same level, the expected range of allowed pressure decay will be reduced or increased, the result will be a more sensitive or less sensitive test. Older pumps with worn valving and debris in the product, usually do not exhibit a good tolerance around the nominal relief pressure settings.
Most flow sensing leak detection devices depend on turbine pumps to supply the constant pressure, for the leak test. The flow sensing device diverts flow from the submersible pump, through an orificed flow detector. With the dispensing nozzle closed, any flow through the flow sensor, is evaluated as a reportable leak, depending on the reporting threshold. The piping system springiness and air entrapped (bulk modulus) is effectively eliminated as a variable in the leak rate determination. Temperature differentials, between the test fluid and the piping remain as critical as with pressure decay devices. The strategies for determining temperature differentials are essentially the same as for pressure decay devices. However, one potentially damaging difference does exist, for pressure decay the pump is off, however, for flow the pump is on. The pump for flow is on, to generate a constant pressure, for the flow test, thereby removing any effect from the system bulk modulus. However, the turbine pump itself, is a source of fluid/piping temperature differential, due to the fluid pulsing caused by the pumps turbine wheel. The additional heat put into the fluid, has the potential to compensate for fluid loss, due to a hole in the piping, by a possible equal expansion of the heated fluid. Remembering that a temperature increase as little as 1 degree centigrade can increase pressure as much as 10 psi for gasoline.
Detecting gross leaks (broken pipe) or leaks as small as 3 gph is easily within the state-of-the-art for most leak detection devices currently available. However for those devices, adding a reliability of performance of 95% detection and 5% false alarm narrows the number of leak detectors that comply. Then add degradation from aging, maintenance and environmental extremes and the number of leak detectors that comply narrows substantially.
Detection of precision leaks in the range of 0.1 to 0.2 gph requires a new level of sophistication. For precision leak detection, a leak detector must compensate for the piping bulk modulus and the temperature differential between fluid and pipe. Existing technology presents pressure decay and flow sensing solutions. However solutions presented currently have failed to recognize, their built in short comings. Therefore more than adequate room is left for additional patents in our opinion.
The following patents are examples of pressure decay line leak detectors:
______________________________________ Reynolds; 3,935,567 January 27, 1976 Elmore III; 4,797,007 January 10, 1989 Michel Et Al; 4,835,717 May 30, 1989 Hill Et Al; 4,876,530 October 24, 1989 Slocum Et Al; 5,103,410 April 7, 1992 ______________________________________
The following patents are examples of flow sensing line leak detectors:
______________________________________ Gerstenmaier Et Al; 4,131,216 December 26, 1978 Maresca, Jr. Et Al; 5,078,006 January 7, 1992 Maresca, Jr. Et Al; 5,090,234 February 25, 1992 ______________________________________
Referring to Reynolds, the line leak detector therein described, fails to compensate for piping bulk modulus, temperature and differing pump off pressures. This device will not reliably find leaks below 3 gph.
Referring to Elmore, III, the line leak detector therein described attempts to compensate for temperature, but fails to incorporate the micro-processor logic necessary for piping bulk modulus and variable pump off pressures.
Referring to Michel Et Al, the line leak detector therein described incorporates a micro-processor to interpret data from a temperature sensor, in order to compensate for fuel/pipe temperature differentials. The device fails to recognize that a single point temperature measurement, does not represent the temperature differentials along the entire length of piping. Additionally, this device fails to compensate for the piping bulk modulus and variable pump off pressures.
Referring to Hill Et Al, the line leak detector therein described incorporates various means to compensate for temperature, bulk modulus and variable pump off pressure, however, their performance is limited to a narrow range, that is insufficient for real world conditions. Temperature differentials between fuel and piping are detected by sequential testing, looking for a decrease in the time to a threshold pressure. When the decrease occurs, the effect is interpreted as due to temperature. Testing is continued until no difference is recorded in the time to the threshold pressure. If the time remains longer than 8 seconds no alarm is activated. This approach ignores the fact that sequential pump operation, itself introduces heat into the fluid, and thereby precludes an accurate measurement. Additionally, leaks below 3 gph cannot be detected in the 8 second time and 5 psi decay. Compensation for differing piping bulk modulus is determined by any time greater than two seconds to achieve 15 psi, after pump shut down. This approach will only work over narrow limits, when the check/relief valves performance is tailored to a specific release rate. Compensation for variable pump off pressure, is attempted to be achieved by a spring loaded piston, delivering make-up fluid to the pressure trapped by the check/relief valve, when the pressure drop is below 12 psi. This device is limited by its range of 4 to 11 psi and its fluid volume of 5 cubic inches. The attempt is to persuade the pump off pressure to always be 10 psi. Considering spring force degradation over time and the make-up volume limit, not to mention the spring force degradation of the pressure relief valve, it certainly is an impossible task.
Referring to Slocum Et Al, the line leak detector therein described incorporates many of the features of Michel Et Al, with some improvements. The primary improvement being an anti-thwart switch, that prevents continuously resetting the leak detector, by laying a brick on the reset switch. This patent does not address or describe any means, whereby any temperature differential, along the entire length of piping, are sensed. A single point or even a multiple fluid temperature sensing points, may miss a section of piping wherein significant temperature gradients may exist. In that situation a probe may indicate no temperature, wherein in fact there is a temperature gradient, further down the pipe which may create significant pressure differentials. A practical example of such a condition would be piping from a dispenser under a canopy. The canopy shades the piping thereunder, but does not shade the piping as it terminates to the underground tank. In that case a temperature sensor under the canopy, would not sense the same temperature as the fuel in the piping that is not shaded, but in fact is warming up. This condition poses a significant restraint to finding a precision leak, with the leak detector herein described. The patent additionally fails to perform the precision leak test on any system that does not have a pump off pressure between 8 to 14 psi. The Slocum leak detector tests from 4 to 3 psi to determine a precision leak. For a pump with an off pressure equal to the pumping pressure, the device would not perform the precision test. For a pump with an off pressure condition that must be compensated for, even at a 3 gph test. This patent mentions the piping bulk modulus, (air) and describes different test times, but fails to relate the test time to a bulk modulus value. For fuel piping in continuous operation, the piping bulk modulus exhibits a constant value. However, the value must be determined, and the leak detector must have a capability, to adjust its performance in accordance with the pipings particular bulk modulus.
Line leak detectors that utilize flow, for piping leak determination, exhibit different performance characteristics than pressure decay devices. However, these devices are as sensitive to the primary obstacle to accurate line leak detection, that is temperature differential, as are pressure decay devices.
Referring to Gerstenmaier Et Al, the line leak detector therein described utilizes a flow detector in a bypass piping arrangement, thereby determining a leak. The test is commenced after 30 minutes of no product pumped through the piping under test. This design is the precursor of later designs using a micro-computer for control. The fundamental fault with this design, relates to temperature stabilization of the fluid under test. The 30 minute wait without pumping product, is insufficient by several order of magnitudes. Additionally flow is determined at a constant pressure, thereby requiring the pump to be on, during the duration of the test. Pulsation of the fluid under test defeats the desired normalized fluid temperature, by introducing heat into the fluid under test.
Referring to Maresca, Jr. Et At, patents dated Jan. 7 and Feb. 25, 1992, the line leak detector therein described, utilizes a modified flow concept. The intent is to eliminate heat induction into the test fluid, during the test cycle. For flow detection methods to work, a constant pressure is required during the flow test. The line leak detector therein described, uses a pressure chamber with an air head, to provide a relatively constant pressure during the flow test. Sets of 3 tests are performed. Flow is determined by the change of height of the test fluid, in the pressure chamber. The first and third tests are flow tested at the selected test pressure, their results are averaged. The second test is performed at zero pressure. It is assumed at zero pressure, no leakage will occur from a potential hole in the piping. The flow if any is then attributed to thermal expansion or contraction of the fluid. Flow again is measured by the change of column height in the test chamber. The average flow from test 1 and 3, is corrected by adding or subtracting test 2, depending on contraction or expansion of the test fluid, from thermal effects. An assortment of valves, pressure chamber, controllers, flow switches, piping, cabling are utilized to obtain the desired performance. The subsequent patent improves with a positive displacement pump and other methods. In addition to the overall complication of this approach, the assumption that fluid will not be lost from a leaking pipe at zero pressure, ignores the inherent pressure head, from the location of the pressure chamber.